This invention relates generally to the field of medical ventilators, and more particularly to an exhalation valve in a ventilator which, when open, permits the venting of respiratory gases from the patient to the atmosphere.
Medical ventilators have been developed to provide artificial respiration to patients whose breathing ability is impaired. Typically, a ventilator will deliver a breath to the patient from a pressurized source of gas. Flow to the patient during inspiration is governed by a flow control valve. When the flow control valve opens, pressurized gas is introduced to the patient's lungs. After the flow control valve closes, ending the inspiration phase of the breath, the patient's respiratory gases are vented to the atmosphere through an exhalation valve, which opens after inspiration is completed and closes before the next inspiration phase begins.
Previous ventilators have been capable of operating in several modes so that the degree of support that the ventilator provides to the patient's natural breathing patterns can be varied. At one extreme, the ventilator can provide fully controlled ventilation in which the ventilator has complete control over when the breath is delivered and the volume of gases received by the patient during each breath. In the fully controlled mode, all of the flow parameters are preset by an operator in accordance with the particular needs of the patient.
At the other extreme, the ventilator can be programmed to permit "spontaneous" breathing by the patient. During the spontaneous breathing mode, the breath rate, the volume of gas inhaled during each breath, and other flow parameters are not predetermined. The inspiration and expiration phases of each breath are commenced in response to efforts by the patient. In between the "volume controlled" and the "spontaneous breath" modes, various degrees of ventilator-supported respiration are available.
One of the parameters which can be controlled by the ventilator during all modes of ventilation is the pressure within the patient's lungs after the expiration is complete. Therapists have found that in some patients, it is beneficial to maintain a slight positive pressure within the lungs after expiration, so as to avoid the possibility of lung collapse. The pressure of the gases in or near the patient's lungs and airway is called the "proximal pressure". Previous ventilators have included a "positive end expiration pressure" (PEEP) feature, which enables the operator to predetermine what the minimum proximal pressure will be after each expiration phase is completed.
To achieve this end, previous ventilators have included microcomputer controllers which "servo" the position of the exhalation valve to maintain the desired proximal pressure level at the end of each breath. In other words, the controller positions the valve based on feedback from a pressure sensor, and causes movement of the valve as is needed to maintain the proximal pressure at the predetermined level.
A difficulty in the design of exhalation valves is that in order to maintain accurate control over the pressure as the valve closes to end the exhalation flow, small amounts of displacement of the valve should not result in large pressure changes. However, the opposite is true when the valve is initially opened at the beginning of exhalation when flow is greater; then it is desirable to have the valve open quickly. This enables a small pressure drop through the valve, thus providing an unobstructed passageway which closely approximates normal breathing, where there is no resistance to exhalation.
One effort to solve this design problem is disclosed in U.S. Pat. No. 4,527,557, in which an exhalation valve is shown which is driven by a combination of electromechanical and pneumatic components. As a result, the valve is complex and expensive to manufacture. Further, due to the lag time associated with pneumatic systems, the valve sacrifices a quick opening capability for the ability to accurately control pressure.
Thus, a need exists for a ventilator having an inexpensive exhalation valve which performs well when the valve needs to open quickly, as well as when the valve is approaching the closed position and is maintaining a predetermined pressure level.